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Need Help with Fox 15 Davis Conversion

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Old 10-11-2012, 09:34 AM
  #51
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Default RE: Need Help with Fox 15 Davis Conversion

Ran the engine today on test stand with puzzling results, started with a old school TopFlite 9-6 Super-M 7300 rpm, switched to a Master Airscrew 9-6 and it would not reach 7000 rpm, I would have thought the MA would have turned faster than the TF with the wide cord. Tried a Zinger 8.5-4 Max rpm was reached with compression screw all the way to the stop, and it would not turn as fast as the TF 9-6.
I am starting to suspect bad fuel I just bought 2.5 Gallons of Davis Fuel from Tower Hobbies but don’t know how long it’s been on their shelf? Also the cans that Davis use now have a plastic spout and cap assembly. You can smell either when you open the bubble wrap that they ship the cans in. I do have some John Deer Starting Fluid (80% either) could I add some of that to the fuel and see if there is improvement? How much?
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Old 10-11-2012, 10:19 AM
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Default RE: Need Help with Fox 15 Davis Conversion

Tweedy its on the bench put the glow head back on run glow with the prop Fox suggest, this will give you a base line, I doubt that you have had any big ether loss from those cans. If your engine still has issues on glow return it to Fox they are great on service, its not the Davis head with the issue martin

I would give Bob Davis a ring first though

a slight odor of ether is normal in the packaging material, do you have an other diesel you can run the fuel in

Also with the Fox on the bench holding the prop and slowly turning does it hold compression

are you sure there is not 2 head gaskets on the engine
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Old 10-11-2012, 12:47 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Put glow head back on, turning 15,000 on 8-4 Kavan, which is right on the Fox specs.

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Old 10-11-2012, 01:13 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Great now put the diesel head back on and see what you get martin
make sure you you do not rotate liner, looks like engine ok
try a 9x5 or 6 then 10x5 then a 10x6 and go from there the whole deal should take less than 15 minutes
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Old 10-11-2012, 02:15 PM
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Default RE: Need Help with Fox 15 Davis Conversion

could only get about 9,000 withthe same 8-4 prop and that was with compression screw all the way in. I returned this head to Davis once because It was leaking around the contra piston, He fitted it with a new contra with a tighter tolerance.
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Old 10-11-2012, 02:20 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Tweedy,

You can check your fuel by the evaporation method.

Leave a measured amount of fuel (say 100ml) in a measuring cylinder an let the ether evaporate. The Kero & Oil will separate and their amounts will be measurable in the cylinder. From this it's easy to calculate the ether content of your fuel, and top up if required.

Greg
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Old 10-11-2012, 02:32 PM
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Default RE: Need Help with Fox 15 Davis Conversion

How long will it take to evaporate?

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Old 10-11-2012, 02:46 PM
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Default RE: Need Help with Fox 15 Davis Conversion

If you stir it maybe 15 minutes tops, one more thought rerun the engine at WOT let it go for at least 5 minutes see if revs pick up then adjust screw and needle if it starts to sound hard slack the compression maybe 1/2 turn or till miss then bring down

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Old 10-11-2012, 02:58 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Maybe the headspace is too big.  It may need a shim removed if the screw is tightened down all the way and still isn't down far enough.  I assume you know more about the dangers of running overcompressed than I do. 
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Old 10-11-2012, 03:07 PM
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Default RE: Need Help with Fox 15 Davis Conversion

looks like I,m at 23% ether
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Old 10-11-2012, 03:27 PM
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Default RE: Need Help with Fox 15 Davis Conversion

You should be able to lower the contra so the edge on the contra is just about in line with the edge of the head IT IS DESIGNED NOT TO PROTRUDE BEYOND THIS POINT-MARTIN also if leaking you would see a good bit of blowby around the compression screw
how many head shims??..Check the edge of the shim with an exacto blade to make sure 2 or not stuck together nd makw sure there is not an other one on the head
tough to see
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Old 10-11-2012, 03:40 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Tweedy

If that is correct, you are low on ether. Look for 30 - 33 percent b/v of ether in your fuel as a good start point.
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Old 10-11-2012, 03:45 PM
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Default RE: Need Help with Fox 15 Davis Conversion

1 Shim, I have tried a new head and get the exact same results.

Richard
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Old 10-11-2012, 03:49 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Would 23% either cause the kind of symptoms I am experiencing?
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Old 10-11-2012, 03:50 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Tweedy open and try an other can of fuel martin
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Old 10-11-2012, 04:12 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Tweedy indeed it would if your number is right 26% about 65cc of John Deere that a bit over 2 fluid oz to the Qt would bring it back martin
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Old 10-11-2012, 05:45 PM
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Default RE: Need Help with Fox 15 Davis Conversion


Quote:
ORIGINAL: aspeed

Maybe the headspace is too big. It may need a shim removed if the screw is tightened down all the way and still isn't down far enough. I assume you know more about the dangers of running overcompressed than I do.
Whoa! Back it up a bit here - if the head space is too big then you would get a misfire big time due to the compression being way too low and if the comp is too low AND the ether content is too low then how did you even get it to start in the first place? Couldn't have been fun.

Something smells very fishy here and it ain't ether.

Check for head leaks as mentioned and get the thing to go minus the muffler firstly.

Good luck.

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Old 10-11-2012, 05:46 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Quote:
ORIGINAL: tweedy

How long will it take to evaporate?

Tweedy
I've found it takes about a day for all the ether to evaporate, and the kero to separate from the oil.

Stirring will not allow the separation for the full analysis of the fuel but will tell you the ether content.

And as previously advised you'll need to have 30-33% ether.


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Old 10-11-2012, 06:00 PM
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Default RE: Need Help with Fox 15 Davis Conversion


I'm guessing that the Fox is used and that the piston/liner is clapped?

Ray
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Old 10-11-2012, 06:05 PM
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Default RE: Need Help with Fox 15 Davis Conversion


Quote:
ORIGINAL: qazimoto


I'm guessing that the Fox is used and that the piston/liner is clapped?

Ray
Good point Ray, and the realigning of the liner in a suspected used engine would not have helped either.

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Old 10-11-2012, 07:21 PM
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Default RE: Need Help with Fox 15 Davis Conversion


Quote:
ORIGINAL: qazimoto


I'm guessing that the Fox is used and that the piston/liner is clapped?

Ray
Engine is new, ran 24 oz. as glow before conversion turning Kavan 8-4 at almost 15,500 and nice idle an transition
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Old 10-11-2012, 07:30 PM
  #72
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Default RE: Need Help with Fox 15 Davis Conversion


Quote:
ORIGINAL: greggles47

Quote:
ORIGINAL: tweedy

How long will it take to evaporate?

Tweedy
I've found it takes about a day for all the ether to evaporate, and the kero to separate from the oil.

Stirring will not allow the separation for the full analysis of the fuel but will tell you the ether content.

And as previously advised you'll need to have 30-33% ether.


I think You are right looked at the sample and it's still evaporating looks like it's going to be in the 30-33% range
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Old 10-11-2012, 07:36 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Is it ever the case that piston to liner fit would be good enough for a engine to run at specs. as a glow but not fit well enough for diesel? wonder if I should order a new piston/liner assembly and just start over?
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Old 10-11-2012, 07:36 PM
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Default RE: Need Help with Fox 15 Davis Conversion

Yes.

However it has not been established that the fit of the piston to liner is the problem. Is compression leaking down significantly when the engine is cold and turned over slowly?

It should hold compression and "push" the piston down as you take it over tdc.

As Mr Davis says in his printed instructions, his conversion head will not perform well with a worn engine.
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Old 10-11-2012, 08:11 PM
  #75
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Default RE: Need Help with Fox 15 Davis Conversion

Quote:
ORIGINAL: fiery

Yes.

However it has not been established that the fit of the piston to liner is the problem. Is compression leaking down significantly when the engine is cold and turned over slowly?

It should hold compression and ''push'' ther piston down as you take it over tdc.

As Mr Davis says in his printed instructions, his conversion head will not perform with a worn engine.
I’m looking now at a bit over 2 hrs. run time between glow and diesel is it possible that it's just not broken in enough yet? It does not seem to have as much compression as my Enya 11 I’m determined to figure this thing out, I built this plane around this engine and all the research I did before making this engine choice told me that it would be a good match for this plane. In fact I’m pretty sure that Enya would fly it.
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